i think that madara plans on possessing Sasuke. i think that Madara, like Oro, wants a new body to possess, but wants to keep his eyes, and his intention his whole time was to push Sasuke to push himself. As for why it had to be Sasuke, maybe it's because Sasuke is like a direct descendant of him or his brother. I may need to think a bit more about and come with an exact reason. When i do i'll edit but i just wanted reflections.